The present invention relates to catalysts for the polymerization of siloxanes and to the use of such catalysts for the polymerization of siloxanes via ring opening of cyclic siloxanes or via condensation of silanol or alkoxy end-blocked siloxane materials or via equilibration of siloxanes. The present invention is more particularly related to phosphazene base catalysts, and especially linear phosphazene base catalysts for said polymerization.
In EP0860461-A, there is described a process for the ring-opening polymerization of cyclosiloxanes, which comprises contacting a cyclosiloxane with 1 to 500 ppm of a phosphazene base, by weight of cyclosiloxane, in the presence of water. The phosphazene base catalyst described in that specification is preferably selected from the general formulae:
((R12)3Pxe2x95x90Nxe2x80x94)x(R12N)3xe2x88x92xPxe2x95x90NR2
{((R12N)3Pxe2x95x90Nxe2x80x94)x(R12N)3xe2x88x92xPxe2x80x94N (H)R2}+(A)xe2x88x92
and
{((R12N)3Pxe2x95x90Nxe2x80x94)y(R12N)4xe2x88x92yP}+(A)xe2x88x92
in which each R1 independently is hydrogen or an optionally substituted hydrocarbon group, or in which two R1 groups bonded to the same N-atom may be linked to complete heterocyclic ring; R2 is hydrogen or an optionally substituted hydrocarbon group; x is 1, 2 or 3; y is 1, 2, 3 or 4; and A is an anion.
EP0879838-A describes a process of preparing a polymer which comprises conducting ring-opening polymerization of a 4- to 10-membered cyclic monomer in the presence of a phosphazenium salt catalyst of an active hydrogen compound represented by the formula 
Although these catalysts are very useful materials, the manufacturing process for many of them is often complex, using in some cases as many as 25 raw materials, solvents and intermediates in up to 12 key process steps, including low temperature manufacture, distillation, filtration, recrystallization and ion-exchange. This makes the process difficult, expensive and time consuming.
We have now surprisingly found that there is no need to make such complex phosphazene base materials for the polymerization of siloxanes. Indeed a more simple structure of phosphazene base materials, in the form of linear polyamino phosphazenium salts containing nucleophilic anions are found to be excellent catalysts for the polymerization of siloxanes, whether by ring-opening polymerization of cyclic siloxanes or by condensation of silanol or silicon-alkoxy end-blocked siloxanes. These linear phosphazene base materials can be made in a much more economic and simple process.
This invention provides a linear phosphazene base catalyst of the general formula, 
wherein R denotes a hydrocarbon having up to 10 carbon atoms or wherein the two R groups on each N atom form with the N atom a heterocyclic group, X denotes an anion and n has an average value of from 1.2 to 10.
This invention also provides a process for making a linear phosphazene base catalyst comprising reacting a linear phosphonitrile halide compound with a compound selected from secondary amines and salts and metal amides thereof to form an aminated phosphazene material, followed by reacting the aminated phosphazene material in an ion exchange reaction replacing the anion of the aminated phosphazene material with a nucleophile.
This invention further provides a process of polymerization of siloxanes via equilibration and/or condensation comprising contacting the siloxanes with the linear phosphazene base catalyst of formula (1).
According to the present invention, a linear phosphazene base catalyst has the general formula 
wherein R denotes a hydrocarbon having 1 to 10 carbon atoms or wherein the two R groups on one N atom form with the N atom a heterocyclic group, X denotes an anion and n is from 1 to 10.
According to one aspect of the invention, a process for making a linear phosphazene base catalyst comprises reacting a linear phosphonitrile halide compound with a secondary amine or a salt of a secondary amine or a metal amide thereof to form an aminated phosphazene material, followed by an ion exchange reaction replacing the anion with a nucleophile.
According to another aspect of the invention, siloxanes are polymerized via equilibration and/or condensation in the presence of a linear phosphazene base catalyst of formula (1).
The process for making the linear phosphazene base catalysts of formula (1) uses as its main ingredient a phosphonitrile halide, preferably a phosphonitrile chloride. Using a phosphonitrile halide, which is already an ionic material, renders the manufacture of the phosphazene base catalysts simpler. Phosphonitrile halides are known and have been described in a number of patent specifications, which are hereby included by reference. Many of these patent specifications describe the use of these halides as catalysts for the polymerization of certain types of siloxanes. For example, GB 910513 discloses phosphonitrile halide catalysts for use in a process for the manufacture of stabilized high viscosity polyorganosiloxane oils and in US 3549680 phosphonitrile halide catalysts are employed in rearrangement reactions. EP 319978 describes chlorophosphonitrile catalysts for use in a process for the preparation of polydiorganosiloxanes containing a silicon-bonded hydroxyl group in each of the terminal units. Other phosphonitrile halide catalysts have been described in GB 2252969. Examples of suitable phosphonitrile halides include (Cl3Pxe2x95x90Nxe2x80x94(Pxe2x95x90NCl2)nxe2x80x94PCl3)+Zxe2x88x92, wherein Zxe2x80x94 is preferably Clxe2x88x92 or PCl6xe2x88x92 and n is 1-10, preferably 1 to 5.
Processes for making these phosphonitrile halide materials are described in many of the above mentioned specifications. One useful process includes the reaction of PCl5 with NH4Cl in the presence of a suitable solvent. In an even more useful process, described in our copending application GB9827055.6, the ammonium chloride is replaced with hexamethyldisilazane. This process comprises a first step of reacting PX5 and a disilazane under cooling in an inert non-chlorinated solvent and a second step of heating the reaction mixture to at least 50xc2x0 C. for a time sufficient to produce the phosphonitrile halide. This process is particularly preferred as the phosphazene halide thus produced has a reduced level of phosphonitrile chloride having only 2 phosphorus atoms present. It has been found that phosphazene bases having only 2 phosphorus atoms, which could be made according to the present invention (formula (1) where the value of n would be 0) are less active as a catalyst for the polymerization of siloxanes.
The above processes generally produce a mixture of phosphonitrile halides; a mixture of phosphonitrile halides can be used as a starting material in the process of the invention, producing a mixture of phosphazene base catalysts. Mixtures of catalysts of formula (1) in which the average value of n is above 1 are preferred catalysts which cannot be prepared by known methods. Particularly preferred linear phosphazene base catalysts are those of formula (1) in which the average value of n is at least 1.2 up to 5 or 10, especially from 1.5 or 1.8 up to 3, as these have been found to be the most effective catalytic species for the polymerization of siloxanes.
The process for preparing the phosphazene base catalyst according to the invention is much less complex than the processes required previously for the making of phosphazene bases, for example those described e.g. by Schwesinger et al. in Liebig. Ann. 1996, 1055-1081. The catalysts according to the present invention can be made by merely reacting the phosphonitrile halide, preferably chloride, with a secondary amine of the formula R2NH, wherein R is selected from hydrocarbon groups having up to 10 carbon atoms, for example methyl, or the two R groups which are found on a single N atom form with the nitrogen atom a heterocyclic group, e.g. a pyrollidine group, a pyrrole group or a pyridine group for the polymerization of siloxanes. Suitable preferred secondary amines include dimethylamine and pyrollidine. Preferably the reaction is carried out in the presence of a material which is able to capture the exchanged halides, e.g. a tertiary amine such as triethylamine. The resulting by-product (e.g. triethyl ammonium chloride) can then be removed from the reaction mixture, e.g. by filtration. The reaction may be carried out in the presence of a suitable solvent for the phosphonitrile chloride and linear phosphazene base. Suitable solvents include aromatic solvents such as toluene.
The amine can be added in its salt form, e.g. dimethylamine hydrochloride, although this is not generally preferred since less common solvents may be required.
The amine can be in the form of its metal amide. For example it can be reacted with an alkali metal or alkyl alkali metal compound, particularly an alkyl lithium compound such as n-butyl lithium, to form the metal amide. After reaction with the phosphonitrile halide, the alkali metal halide can be removed by filtration.
The groups R in the linear ionic phosphazene base catalyst of formula (1) are derived from the secondary amine R2NH. The two R groups on each N atom can be the same or different, e.g. the secondary amine can be methylethylamine. The NR2 groups in the phosphazene base are generally the same as each other although they can be different if a mixture of secondary amines is used.
The linear phosphazene material which is formed this way is generally in the form of a salt with the anion Zxe2x80x94, e.g. Clxe2x80x94 or PCl6xe2x80x94, derived from the phosphonitrile halide. This is preferably treated in an ion exchange reaction (preferably by passing over an ion exchange resin) whereby the anion is replaced with a basic nucleophile Xxe2x80x94 to form a stronger base. The counterion X is preferably hydroxyl (hydroxide), an alkoxy (alkoxide) group having up to 25 carbon atoms, or silanolate, fluoride, carbonate or bicarbonate. The phosphazene is preferably dispersed in a suitable medium prior to passing through an ion exchange system. Suitable media include water, alcohol and mixtures thereof.
Polymerization of siloxanes according to the present invention includes equilibration polymerization, ring opening polymerization of cyclic siloxanes, and condensation polymerization, which is based on the condensation of silanol groups, which may be present in the siloxanes, or which may be formed there in situ, e.g. by hydrolysis of silicon-bonded hydrolyzable groups such as alkoxy groups.
The starting material for ring-opening polymerization is a cyclosiloxane (also known as a cyclic siloxane). Cyclic siloxanes which are useful are well known and commercially available materials. They have the general formula (R32SiO)m, wherein R3 denotes hydrogen or an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group having up to 8 carbon atoms, m denotes an integer with a value of from 3 to 12. R3 can be substituted, e.g. by halogen such as fluorine or chlorine. The alkyl group can be, for example, methyl, ethyl, n-propyl, trifluoropropyl, n-butyl, sec-butyl, and tert-butyl. The alkenyl group can be, for example, vinyl, allyl, propenyl, and butenyl. The aryl and aralkyl groups can be, for example, phenyl, tolyl, and benzoyl. The preferred groups are methyl, ethyl, phenyl, vinyl, and trifluoropropyl. Preferably at least 80% of all R3 groups are methyl or phenyl groups, most preferably methyl. It is most preferred that substantially all R3 groups are methyl groups. Preferably the value of m is from 3 to 6, most preferably 4 or 5. Examples of suitable cyclic siloxanes are octamethyl cyclotetrasiloxane, decamethyl cyclopentasiloxane, cyclopenta (methylvinyl) siloxane, cyclotetra (phenylmethyl) siloxane and cyclopenta methylhydrosiloxane. One particularly suitable commercially available material is a mixture of octamethylcyclo-tetrasiloxane and decamethylcyclopentasiloxane.
The starting material for equilibration polymerization can be cyclic siloxanes as described above and/or any polydiorganosiloxane material having units of the general formula Rxe2x80x2aSiO4-a/2 (2) wherein Rxe2x80x2 denotes a hydrogen atom, a hydrocarbon group having from 1 to 18 carbon atoms, a substituted hydrocarbon group having from 1 to 18 carbon atoms or a hydrocarbonoxy group having up to 18 carbon atoms and a has on average a value of from 1 to 3, preferably 1.8 to 2.2. Preferably the polydiorganosiloxanes are polydialkylsiloxanes, and most preferably polydimethylsiloxanes. They are preferably substantially linear materials, which are end-blocked with a siloxane group of the formula Rxe2x80x33SiO1/2, wherein Rxe2x80x3 is Rxe2x80x2 or hydroxyl.
Starting materials for the condensation reaction of silanol containing siloxanes are polyorganosiloxanes having silicon-bonded hydroxyl groups or hydrolyzable groups such as alkoxy groups, which may form silanol groups in situ. These include, for example, polyorganosiloxanes having the a general formula (3): 
In formula (3), R3 denotes hydrogen or an optionally substituted alkyl, alkenyl, aryl, alkaryl or aralkyl group having up to 8 carbon atoms, each R4 denotes a monovalent hydrocarbon group preferably having 1 to 18 carbon atoms or halogenated hydrocarbon group preferably having 1 to 18 carbon atoms and t is an integer having a value of from at least 2. Preferably R4 denotes an alkyl group having from 1 to 6 carbon atoms and more preferably a methyl group. The value of t is preferably such that the average viscosity of the polyorganosiloxanes does not exceed 200 mm2/s at 25xc2x0 C.
Polyorganosiloxanes having terminal silicon-bonded hydroxyl groups are well known and are commercially available. Preferred polyorganosiloxanes have one silicon-bonded hydroxyl group in each terminal group and have at least 80% of the R4 groups denote a methyl group. Suitable polyorganosiloxanes for use as reagents in a polymerization process in which the catalysts of the invention are used include polydiorganosiloxanes having terminal hydroxydiorganosiloxane units, e.g. hydroxyldimethyl siloxane end-blocked polydimethylsiloxanes, hydroxyldimethyl siloxane end-blocked polydimethyl poly-methylphenyl siloxane copolymers, triorganosiloxane end-blocked polydimethylsiloxanes, e.g. trimethylsiloxane end-blocked polydimethylsiloxanes and cyclic polydiorganosiloxanes, e.g. polydimethylcyclosiloxanes.
Thus the process according to the invention is useful for making polyorganosiloxanes having units of the general formula Rxe2x80x2aSiO4a/2 (2) wherein Rxe2x80x2 and a are as described above. Preferably at least 80% of all Rxe2x80x2 groups are alkyl or aryl groups, more preferably methyl groups. Most preferably substantially all Rxe2x80x2 groups are alkyl or aryl groups, especially methyl groups. The polyorganosiloxanes are preferably those in which the value of a is 2 for practically all units, except for the end-blocking units, and the siloxanes are substantially linear polymers of the general formula Rxe2x80x3(Rxe2x80x22SiO)pSiRxe2x80x22Rxe2x80x3 (3) wherein Rxe2x80x2 and Rxe2x80x3 are as defined above and p is an integer. It is, however, also possible that small amounts of units wherein the value of a denotes 0 or 1 are present. Polymers with such units in the chain would have a small amount of branching present. Preferably Rxe2x80x3 denotes a hydroxyl group or an alkyl or aryl group, e.g. methyl or phenyl. The viscosity of the polyorganosiloxanes which may be produced by the process using a catalyst according to the present invention may be in the range of from 1000 to many millions mm2/s at 25xc2x0 C., depending on the reaction conditions and raw materials used in the method of the invention.
The process according to the invention can be used to make a whole range of siloxane polymers, including liquid siloxane polymers and gums of high molecular weight, for example from 1xc3x97106 to 100xc3x97106. The molecular weight of silicone polymers is effected by the concentration of end groups and in the absence of added end groups is determined by the catalyst concentration. The catalyst used in the present invention has sufficient activity to enable the formation of polymers in a reasonable time at a low catalyst concentration.
We have found that phosphazene base catalysts according to the invention when used at very low concentrations (2-500ppm) based on the weight of the cyclosiloxanes produce polymers with very high molecular weights (1,000,000-100,000,000) very quickly (10 seconds to 8 hours) even at moderate to low temperatures (20-170xc2x0 C.). Molecular weight changes during polymerization can be monitored by sampling the reaction during polymerization, and analyzing each sample by gel permeation chromatography to determine the molecular weight. Polymers of very high molecular weights can be obtained because of the very low catalyst concentrations needed for the polymerization, with the result that the molecular weight of the polymer produced is dependent on the end group concentration which is equal to the catalyst concentration. However, we have found that at very low catalyst concentrations, such as 2 ppm, the molecular weight obtained increases with reaction time. The process may be limited by diffusion of the catalyst, which is very slow in these high molecular weight polymers.
As an alternative to high molecular weight gums, the process according to the invention can also be used in equilibration reactions to produce silicone fluids, for example in the viscosity range at 25xc2x0 C. of from 1 to 150,000 mm2/s. An end-blocker is added in a proportion calculated to produce the desired molecular weight of polymer. Suitable end-blockers are, for example, polysiloxanes, particularly linear polysiloxanes, in the molecular weight range from 160 upwards, in particular polydimethylsiloxanes of the general formula MDxM where M is (CH3)3SiO1/2, D is xe2x80x94Si(CH3)2O2/2xe2x80x94 and x has a value of from 0 to 20. The end-blocker may have one or more functional groups such as hydroxyl, vinyl or hydrogen. Water also acts as a end-blocker, with the introduction of hydroxyl functional groups. We have found that the use of an end-blocker gives better control of the molecular weight and polydispersity of the siloxane polymer produced than other methods of control such as stopping the polymerization reaction before completion.
The catalysts of the invention may be used at a concentration of from 1 to 500 ppm by weight based on the total weight of the organosiloxanes used as reagents in a polymerization process. Preferably from 5 to 150 ppm by weight are used, most preferably from 5 to 50 ppm. The amount of catalyst used in the method of the invention may be reduced when the temperature at which the organosilicon compounds and the catalyst are contacted is increased. The method of the invention may conveniently be carried out at room temperature or at temperatures as high as 250xc2x0 C. or 300xc2x0 C. or even higher. The preferred temperature range may be from 50 to 170xc2x0 C.
The polymerization can be carried out in bulk or in the presence of a solvent. Suitable solvents are liquid hydrocarbons or silicone fluids. The phosphazene base catalyst can be diluted in a solvent such as toluene or dispersed in a silicone fluid such as polydiorganosiloxanes.
The polymerization reaction can be carried out at ambient temperature or under heating. Heating, for example to 100xc2x0 C. or higher, is appropriate when the catalyst activity has been moderated as described below. The time taken for polymerization will depend on the activity of the catalyst in the chosen system, and on the desired polymer product. In the absence of moderation, the phosphazene base catalysts are sufficiently active to convert cyclosiloxanes such as octamethylcyclotetrasiloxane to high molecular weight polysiloxane gums within a few seconds.
When the desired polymer has been formed, it is usually desirable to neutralize the catalyst to stabilize the product and prevent any further reaction. Suitable neutralizing agents are acids such as acetic acid, silyl phosphate, polyacrylic acid chlorine substituted silanes, silyl phosphonate or carbon dioxide.
We have found during preparation of the phosphazene base catalysts that air reacts very rapidly with the catalyst solutions giving a hazy material which eventually leads to an insoluble liquid phase. This is believed to be due to the reaction of the catalyst with CO2 to form a carbonate salt. We have also found that this deactivation of the catalyst can be reversed e.g. by heating, purging with inert gas or subjecting the mixture to reduced pressure. This makes it possible to moderate or control the polymerization reaction. This is particularly advantageous in view of the very rapid reaction which occurs when the catalyst is not moderated. Because of the very low levels of catalyst employed in these reactions (which can be as low as 1-10 ppm), the reaction with CO2 needs to be taken into account to control the reaction and obtain reproducible results.
We have also found that polymerization can be prevented by exposing a mixture of siloxane and phosphazene base catalyst to air and/or CO2 or larger amounts of water. The polymerization can then be initiated (xe2x80x9ccommand polymerizationxe2x80x9d) simply by removing the air and/or CO2 or the water e.g. by heating the mixture (e.g. to 100xc2x0 C.-170xc2x0 C. for a few minutes).